Subsea heat exchanger

ABSTRACT

Subsea convection heat exchanger ( 1 ) for cooling or heating a hydrocarbon-containing fluid in subsea environment. The heat exchanger ( 1 ) comprises a convection section with a fluid carrying pipe ( 7 ) adapted for heat transfer between the carried fluid on one side of the pipe wall and the surrounding water on the opposite side of the pipe wall. The convection section is enclosed by an enclosure ( 9 ) with sea water inlet ( 11 ) and a sea water outlet ( 13 ). The heat exchanger ( 1 ) is furthermore provided with means ( 15 ) for controlled through-flow of surrounding sea water from the sea water inlet ( 11 ) to the sea water outlet ( 13 ).

The present invention relates to subsea processing of hydrocarbon-containing fluids. In particular the invention relates to a convection heat exchanger adapted to operate in a body of surrounding water, using said water as cooling or heating medium.

BACKGROUND

In subsea fluid processing it is known to use the surrounding seawater to cool or heat fluid flowing in a pipe. Common practice is to arrange said pipe with a plurality of bends or combining a plurality of such pipes in a parallel configuration in order to achieve large contact area between the pipe and the water, and thus a high heat transfer rate between the fluid in the pipe(s) and the surrounding water. If there is a current present in the seawater, the heat transfer rate will rise. But even with no current, the pipe and its interior fluid will be cooled or heated in dependence of the temperature difference between the fluid and the surrounding seawater.

In processes where the flow-rate, the temperature, or characteristics of the fluid flowing through the pipe varies, the above described arrangement can involve challenges for the operator, since he cannot control the exact cooling or heating rate. Varying temperatures of the surrounding seawater can also imply corresponding challenges.

Possible remedies for such challenges can be to control the flow rate of the fluid in the pipe(s) or to flow the fluid through longer or shorter lengths of the pipe by the control of appropriately arranged valves. For instance, the convection section can consist of a plurality of parallel branches. If a higher heat transfer rate is needed, more branches can be connected. Correspondingly, if less heat transfer rate is needed, the operator can disconnect one or more branches.

However, each of the above solutions exhibits a disadvantage. Depending on the specific process in question, controlling the flow rate of the fluid can in many cases not be done because it increases complexity, costs, and lowers reliability. Dividing the flow into a specific number of branches by the use of valves will limit the heat transfer rate into a limited number of possible heat transfer rate values, depending on the number and the design/size of each branch.

Thus, there is a need for a subsea convection heat exchanger that overcomes the above-mentioned disadvantages. Such a heat exchanger is provided with the subsea convection heat exchanger according the present invention, as given in the characteristic part of the independent claim 1.

The Invention

According to the invention, there is provided a subsea convection heat exchanger for cooling or heating a hydrocarbon-containing fluid in a subsea environment. The heat exchanger comprises a convection section with a fluid carrying pipe adapted for heat transfer between the carried fluid on one side of the pipe wall and the surrounding water on the opposite side of the pipe wall. The convection section is enclosed by an enclosure comprising a sea water inlet and a sea water outlet. Furthermore, the heat exchanger is provided with means for controlled through-flow of surrounding sea water from the sea water inlet to the sea water outlet. With such a subsea convection heat exchanger, an operator is able to control the heat transfer rate between the fluid flowing through the convection section and the sea water. This can be performed for instance by varying the pump speed or by controlling a throttling valve to control the through-flow of sea water.

Preferably, the heat exchanger according to the invention is hydrostatically balanced, as there is fluid connection between the exterior and interior of the enclosure, preferably through said inlet and/or outlet. Thus, it can operate in any depth as desired without having to dimension the enclosure according to hydrostatic pressure in the surrounding sea water.

The means for through-flow of sea water is advantageously a pump. The pump can be arranged in connection with the sea water outlet of the enclosure, thereby creating a pressure drop from the exterior of the enclosure to its interior.

In a particularly preferred embodiment, a pump is arranged in a pump unit with a pump motor arranged in a motor chamber (23) which is isolated from the surroundings and adapted to be hydrostatically balanced with surrounding sea water. The motor is then preferably adapted to operate the pump with a shaft running through a mechanical seal, and in such way that the arrangement of the pump unit creates a pressure drop over said mechanical seal from the motor chamber to the pump when the pump is running, as the pump is adapted to create a pressure drop from its outlet to its inlet. This advantageous set-up results in that sea water does not penetrate into the motor chamber through the seal, thereby contributing to a longer operating time of the pump motor.

In an advantageous embodiment, the heat exchanger according to the invention preferably exhibits a fluid inlet and a fluid outlet for the fluid to be cooled or heated, wherein the means for through-flow of water is controlled by a controller which at least partially is controlled on the basis of the temperature of the fluid flowing in the fluid outlet and/or inlet, as measured by temperature sensor(s). The control of the means for through-flow of sea water can then be automatically controlled.

EXAMPLE OF EMBODIMENT

In the following, an example of an embodiment of the subsea convection heat exchanger according to the present invention will be given with reference to the drawings, in which

FIG. 1 is a diagrammatic view of a subsea convection heat exchanger; and

FIG. 2 is a diagrammatic view of a preferred arrangement of the pump and pump motor.

FIG. 1 illustrates a subsea convection heat exchanger 1 according to the present invention. It is arranged on the seabed in connection with a subsea processing facility. The purpose of the heat exchanger 1 is to obtain exact control of process outlet temperature of a hydrocarbon-containing gas in order to avoid condensation or the formation of hydrates.

The heat exchanger 1 exhibits a high pressure inlet 3 for process fluid, and a high pressure process outlet 5. The heat exchanger 1 further exhibits a convection section comprising a plurality of heat transfer pipes 7. The heat transfer pipes 7 are in direct contact with the sea water.

The convection section of the heat exchanger 1 is encapsulated with an enclosure 9. The enclosure has a sea water inlet 11 and a sea water outlet 13. In connection with the sea water outlet 13, there is arranged a remotely controllable pump unit 15.

The pump unit 15 is described in more detail below with reference to FIG. 2. Still referring to FIG. 1, the pump unit 15 is adapted to pump ambient sea water into the enclosure 9 through the sea water inlet 11 and out of the enclosure 9 through the sea water outlet 13. As described in more detail below, the pump unit 15 is advantageously arranged on the downstream side of the sea water flow, in connection with the sea water outlet 13.

The assembly of the pump unit 15 and enclosure 9 which encapsulates the convection section, results in an exact process control of the temperature of the process fluid flowing out of the heat exchanger 1 through the process fluid outlet 5. As the controllable pump unit 15 flows surrounding sea water through the heat exchanger 1, the controlling of the pump unit 15 will directly control the heat transfer rate of the heat exchanger. In addition, if no convection or heat transfer is desired, the pump unit 15 can be halted, and as the sea water inside the enclosure 9 approaches the temperature of the process fluid, practically no heat transfer will take place. In order to reduce heat transfer through the enclosure 9 between the sea water inside and outside of the enclosure 9, one can arrange additional enclosures or isolated enclosures (not shown).

Advantageously, the pressure difference between the enclosure interior and the surrounding sea water needs not be much in order to flow a desired amount of sea water through the heat exchanger 1. Thus, only a small pump unit 15 is needed.

Preferably, a temperature sensor (not illustrated) is arranged to measure the fluid temperature of the process fluid flowing out of process fluid outlet 5. To monitor the operation of the heat exchanger 1, one can preferably also arrange a temperature sensor (not illustrated) in connection with the process fluid inlet 3.

FIG. 2 illustrates the pump unit 15 in more detail. The pump unit 15 comprises a pump motor 17 and a pump 19 arranged in a pump unit housing 21. The pump motor 17 is arranged in a motor chamber 23 which is isolated from the surroundings, but which is hydrostatically balanced with the pressure of the surrounding sea water (preferably with a membrane solution). The motor 17 operates the pump 21 with a shaft running through a mechanical seal 25. As mentioned above, the pump unit 15 is advantageously arranged on the downstream side of the sea water flow. Such an arrangement results in a pressure drop over the mechanical seal 25 from the motor side to the pump side. Thus, no sea water will penetrate into the motor chamber 23 along the mechanical seal 25.

In another embodiment of the present invention, the means to provide for the flow of sea water through the heat exchanger is not a regular pump. Instead, the heat exchanger is arranged in such manner that the sea water will flow through it in a vertically direction. Due to the heat transfer in the convection section, the temperature difference between the sea water inside and outside the enclosure will result in different specific gravity of the water. This difference will effect the vertical flow of sea water. The flow can for instance be controlled by a valve at the inlet of outlet of sea water. 

1. A subsea convection heat exchanger for cooling or heating a hydrocarbon-containing fluid in a subsea environment, said heat exchanger comprising: a convection section comprising a fluid carrying pipe for heat transfer between carried fluid on one side of a pipe wall and surrounding water on an opposite side of the pipe wall; wherein said convection section is enclosed by an enclosure comprising a sea water inlet and a sea water outlet; and wherein the subsea convection heat exchanger comprises a pump for controlled through-flow of surrounding sea water from the sea water inlet to the sea water outlet.
 2. A subsea convection heat exchanger according to claim 1, wherein the subsea convection heat exchanger is hydrostatically balanced as there is fluid connection between an exterior and interior of the enclosure through at least one of said sea water inlet and said sea water outlet.
 3. (canceled)
 4. A subsea convection heat exchanger according to claim 1, wherein said pump is arranged in connection with the sea water outlet of the enclosure creating a pressure drop from the exterior of the enclosure to the interior of the enclosure.
 5. A subsea convection heat exchanger according to claim 4, wherein: said pump is arranged in a pump unit with a pump motor arranged in a motor chamber which is isolated from the surroundings and adapted to be hydrostatically balanced with surrounding sea water; wherein the pump motor is adapted to operate the pump via a shaft running through a mechanical seat; and wherein the arrangement of the pump unit creates a pressure drop over said mechanical seal from the motor chamber to the pump when the pump is running, as the pump is adapted to create a pressure drop from its outlet to its inlet.
 6. A subsea convection heat exchanger according to claim 1, wherein: the subsea convection heat exchanger exhibits a fluid inlet and a fluid outlet for the fluid to be cooled or heated; and wherein said pump for through-flow of water is controlled by a controller, wherein the controller is at least partially controlled on a basis of a temperature of the fluid flowing in at least one of the fluid outlet and the fluid inlet, as measured by temperature sensor(s). 